Absorption and/or desorption are processes that are exploited in many processes. These processes involve the gain or loss of a component to a solution. Many factors affect these processes, including solution compositions, heat exchange, surface area, temperatures, and other factors. Many absorption and desorption processes involve aqueous solutions.
Absorption of species into a liquid is widely used in many technologies, including: absorption heat pumps; liquid desiccant-based dehumidification; purification of the natural gas streams involving separation of CO2 and H2S; removal of CO2 from flue gas; and bioreactors in which gaseous products are absorbed into a liquid phase for processing/conversion by microorganisms. Absorption is limited by the rate of absorbate diffusion into the absorbent. In cases involving an absorbate with a high heat of phase change, such as water vapor absorption into a lithium bromide (LiBr) solution in an absorption heat pump or a dehumidifier, the absorption rate is also limited by thermal diffusion. When the heat released at the vapor-liquid interface due to phase change is not removed, there is an increase in temperature and equilibrium water vapor pressure at the interface. This increase in water vapor pressure lowers the rate of water vapor absorption.
Enhancement of water vapor absorption rates into LiBr have been pursued, yet few tangible results have been achieved due to the challenges of controlling thermohydraulic characteristics of a falling LiBr solution film. Yu et al., “Parametric Study of Water Vapor Absorption Into a Constrained Thin Film of Lithium Bromide Solution” International Journal of Heat and Mass Transfer 2012, 55, 5687-95 discloses a numerical model for the absorption characteristics of LiBr solution flows and that heat and mass transfer limits in a LiBr solution flow could be enhanced in flow channels with thicknesses less than a few hundred microns when employing high flow velocities. Thin solution high velocity flow reduces the heat diffusion path to the cooling surface beneath the solution film and a high solution flow velocity diminishes the thickness of any concentration boundary layer. Nasr et al. “Absorption Characteristics of Lithium Bromide (LiBr) Solution Constrained by Superhydrophobic Nanofibrous Structures” International Journal of Heat and Mass Transfer 2013, 63, 82-90 disclosed an experimental study where a LiBr solution flow is constrained by a superhydrophobic nanofibrous membrane with the absorption characteristics could be modified by control of the flow thickness and flow velocity to significantly increase absorption rates over those of falling films, where a solution film thicknesses on the order of about 100 μm was found to give superior results. However, the manifolding burden of an absorber having such a thin solution flow channel is high and tends to limit the capacity of the absorber. Hence, one is typically obliged to reduce the flow pressure drop by increasing the solution channel thickness, as flowing a fixed amount of mass through a 100 μm thick channel has approximately a 125 times greater pressure drop than that of a 500 μm thick channel. However, increasing the solution channel thickness also reduces the absorption rate, since transport within the solution flow as molecular diffusion is the sole mechanism for the absorbed molecule to be transported into the bulk absorbent, and mixing is needed to achieve high absorption rates.
Common desorbers for absorption refrigeration systems (ARSs) involve nucleate pool boiling or falling films over horizontal or vertical tubes which are the common configurations in lithium bromide (LiBr). In the pool boiling configuration, water is boiled from a pool of LiBr solution. In a falling film desorber, a LiBr solution is sprayed over a tube bundle while the heating medium flows inside the tubes. At low surface temperatures, water directly diffuses from the solution film when the solution temperature remains sufficiently high to sustain a solution water vapor pressure above the external vapor pressure. When the wall temperature sufficiently exceeds the solution saturation temperature, desorption also takes place from bubble nuclei formed at the solid-liquid interface.
A superheat temperature, which is the difference between the wall and solution saturation temperatures, of approximately 10° C. is required for boiling inception. Water bubble growth rate is significantly slow in the LiBr solution because of the low water diffusion coefficient in the solution. As in pure water, bubble growth in the LiBr solution is limited by mass diffusion rather than by heat transfer. Consequently, a significant surface superheat temperature is required to grow bubbles where the buoyancy force overcomes the surface tension and departure from the heat transfer surface.
An increase in the desorption rate and a reduction of the required surface superheat temperature are desirable for reducing the size of a desorber and lowering its heating medium temperature. For example, the use of an absorption cycle with solar-thermal collectors or photovoltaics with waste heat recovery is benefitted by a reduction of the required heat source temperature to enhance the prospect of directly converting solar heat into a cooling effect.
Mixing has been examined in laminar flow microchannels and involves inclusion of active or passive components. Active mixers, such as piezoelectric and magneto/electrohydrodynamic actuators or pressure perturbers place additional burden on the system. A passive approach is disclosed in Stroock et al. Chaotic Mixer for Microchannels” Science 2002, 295, 647-51 where chaotic advection is generated within the flow through stretching and folding the laminar streamlines due to ridges formed in the base of a channel. This passive mixture was employed to mix streams of liquids that are introduced at an inlet to an enclosed channel. The effect of such mixers on absorbers and desorbers where a vapor is included or excluded over the length of the channel has not been examined.